Design principles of dual-functional molecular switches in solid-state tunnel junction

Molecular electronics has improved tremendously over the past 20 years, but it remains challenging to develop molecular switches that operate well in two-terminal tunnel junctions. Emerging technologies demand multi-functional junctions that can switch between different operations within a single molecule or molecular monolayer. Usually the focus is placed on molecules that shift the junctions between high and low conductance states, but here we describe molecular junctions with dual-functional switching capability. We discuss the operating mechanism of such switches and present examples of “two-in-one” junctions of a diode placed in series with an additional switch, which can operate either as an electrostatic or a memory on/off switch. We propose guidelines for future designs of such dual-function molecular switches and provide an outlook for future directions of research

Molecular Coatings for Stabilizing Silver and Gold Nanocubes under Electron Beam Irradiation

We study the degradation process of closely spaced silver and gold nanocubes under high-energy electron beam irradiation using transmission electron microscopy (TEM). The high aspect ratio gaps between silver and gold nanocubes degraded in many cases as a result of protrusion and filament formation during electron beam irradiation. We demonstrate that the molecular coating of the nanoparticles can act as a protective barrier to minimize electron-beam-induced damage on passivated gold and silver nanoparticles

Molecular Coatings for Stabilizing Silver and Gold Nanocubes under Electron Beam Irradiation

We study the degradation process of closely spaced silver and gold nanocubes under high-energy electron beam irradiation using transmission electron microscopy (TEM). The high aspect ratio gaps between silver and gold nanocubes degraded in many cases as a result of protrusion and filament formation during electron beam irradiation. We demonstrate that the molecular coating of the nanoparticles can act as a protective barrier to minimize electron-beam-induced damage on passivated gold and silver nanoparticles

Equivalent Circuits of a Self-Assembled Monolayer-Based Tunnel Junction Determined by Impedance Spectroscopy

The electrical characteristics of molecular tunnel junctions are normally determined by DC methods. Using these methods it is difficult to discriminate the contribution of each component of the junctions, e.g., the molecule–electrode contacts, protective layer (if present), or the SAM, to the electrical characteristics of the junctions. Here we show that frequency-dependent AC measurements, impedance spectroscopy, make it possible to separate the contribution of each component from each other. We studied junctions that consist of self-assembled monolayers (SAMs) of <i>n</i>-alkanethiolates (S­(CH₂)_<i>n</i>−1CH₃ ≡ SC_<i>n</i> with <i>n</i> = 8, 10, 12, or 14) of the form Ag^TS-SC_<i>n</i>//GaO_<i>x</i>/EGaIn (a protective thin (∼0.7 nm) layer of GaO_<i>x</i> forms spontaneously on the surface of EGaIn). The impedance data were fitted to an equivalent circuit consisting of a series resistor (<i>R</i>_S, which includes the SAM-electrode contact resistance), the capacitance of the SAM (<i>C</i>_SAM), and the resistance of the SAM (<i>R</i>_SAM). A plot of <i>R</i>_SAM vs <i>n</i>_C yielded a tunneling decay constant β of 1.03 ± 0.04 <i>n</i>_C^–1, which is similar to values determined by DC methods. The value of <i>C</i>_SAM is similar to previously reported values, and <i>R</i>_S (2.9–3.6 × 10^–2 Ω·cm²) is dominated by the SAM–top contact resistance (and not by the conductive layer of GaO_<i>x</i>) and independent of <i>n</i>_C. Using the values of <i>R</i>_SAM, we estimated the resistance per molecule <i>r</i> as a function of <i>n</i>_C, which are similar to values obtained by single molecule experiments. Thus, impedance measurements give detailed information regarding the electrical characteristics of the individual components of SAM-based junctions

The Origin of the Odd–Even Effect in the Tunneling Rates across EGaIn Junctions with Self-Assembled Monolayers (SAMs) of <i>n</i>‑Alkanethiolates

Odd–even effects in molecular junctions with self-assembled monolayers (SAMs) of <i>n</i>-alkanethiolates have been rarely observed. It is challenging to pinpoint the origin of odd–even effects and address the following question: are the odd–even effects an interface effect, caused by the intrinsic properties of the SAMs, or a combination of both? This paper describes the odd–even effects in SAM-based tunnel junctions of the form Ag^A‑TS-SC_<i>n</i>//GaO_<i>x</i>/EGaIn junctions with a large range of molecular lengths (<i>n</i> = 2 to 18) that are characterized by both AC and DC methods along with a detailed statistical analysis of the data. This combination of techniques allowed us to separate interface effects from the contributions of the SAMs and to show that the odd–even effect observed in the value of <i>J</i> obtained by DC-methods are caused by the intrinsic properties of the SAMs. Impedance spectroscopy (an AC technique) allowed us to analyze the SAM resistance (<i>R</i>_SAM), SAM capacitance (<i>C</i>_SAM), and contact resistance, within the junctions separately. We found clear odd–even effects in the values of both <i>R</i>_SAM and <i>C</i>_SAM, but the odd–even effect in contact resistance is very weak (and not responsible for the observed odd–even effect in the current densities obtained by <i>J</i>(V) measurements). Therefore, the odd–even effects in Ag^A‑TS-SC_<i>n</i>//GaO_<i>x</i>/EGaIn junctions are attributed to the properties of the SAMs and SAM–electrode interactions which both determine the shape of the tunneling barrier

Direct measurement of the local field within alkyl-ferrocenylalkanethiolate monolayers: Importance of the supramolecular and electronic structure on the voltammetric response and potential profile

This paper describes the electrochemical behaviour of self-assembled monolayers (SAMs) of n-alkanethiolates with Fc groups inserted at 14 different positions along the alkyl chain (SCnFcC13-n, n ¼ 0e13) studied by cyclic voltammetry. The electronic and supramolecular structures of the SAMs have been fully characterised and all molecules are standing up, allowing for precise control over the position of the Fc unit within the SAM as a function of n revealing the shape of the electrostatic potential profile across the SAMs. The potential profile is highly non-linear due to electronic changes in the nature of the Fcdelectrode interaction for small values of n < 5, and supramolecular changes for large values of n ¼ 11 e13. For intermediate values of n ¼ 5e11, the potential drop is linear and the data can be fitted to a model developed by White and Smith. The electrochemical behaviour was dominated by a one-step reversible redox-process, but the presence of a shoulder indicates that the Fc units are present in different microenvironments resulting from the mismatch in size between the Fc units and the alkyl chains. Other features, including peak splitting, peak broadening, and peak shifts, can be related to changes in the electronic and supramolecular structure of the SAM revealed by molecular dynamics simulations and spectroscopy. For small values of n < 5, electronic effects dominate and the peak oxidation waves are shifted anodically (~150 mV) and broadened (full width at half maximum of up to 220 mV) because the Fc units hybridise with the Au electrode (for n < 3) or interact with the Au electrode via van der Waals interactions (n ¼ 4, 5). For intermediate values of n ¼ 5e11, supramolecular effects direct the packing structure of the SAMs and clear odd-even effects are observed. For large values of n ¼ 11e13, the top alkyl chains are liquid-like in character and do not block the Fc units from the electrolyte

One Carbon Matters: The Origin and Reversal of Odd–Even Effects in Molecular Diodes with Self-Assembled Monolayers of Ferrocenyl-Alkanethiolates

We investigated the origin of odd–even effects in molecular diodes based on self-assembled monolayers (SAMs) of ferrocenyl-terminated <i>n</i>-alkanethiolates S­(CH₂)_<i>n</i>Fc with <i>n</i> = 6–15 on Ag or Au surfaces contacted with EGaIn top electrodes. These SAMs have different M–S–C bond angles of 180° when M = Ag and 104° when M = Au causing a multitude of odd–even effects in the performance of the diodes. By changing the M–S–C bond angles and using several characterization techniques, we were able to systematically identify and rationalize odd–even effects in the electronic structure of the device. Changing <i>n</i> from 6 to 15 resulted in an odd–even effect in the tilt angle of the Fc units (α), which, in turn, caused odd–even effects in the surface dipole, work function, and HOMO onset (HOMO = highest occupied molecular orbital). These odd–even effects caused an odd–even modulation of the tunneling current across the diode in the on state (the current that flows across the junctions when the diode allows the current to pass through). The current that flows across the diodes in their off state (the leakage current) also followed an odd–even effect that was related to an odd–even effect in the packing energy: SAMs with small tilt angles of the ferrocenyl units α (with the Fc units standing up) pack better than SAMs with large α values (with the Fc units in a parallel orientation with the plane of the electrode). All these odd–even effects were completely and consistently reversed when the Ag electrodes were replaced with Au electrodes proving they are induced by the M–S–C bond angle

A Molecular Diode with a Statistically Robust Rectification Ratio of Three Orders of Magnitude

This paper describes a molecular diode with high, statistically robust, rectification ratios <i>R</i> of 1.1 × 10³. These diodes operate with a new mechanism of charge transport based on sequential tunneling involving both the HOMO and HOMO–1 positioned asymmetrically inside the junction. In addition, the diodes are stable and withstand voltage cycling for 1500 times, and the yield in working junctions is 90%

Energy-level alignment and orbital-selective femtosecond charge transfer dynamics of redox-active molecules on Au, Ag, and Pt metal surfaces

Charge transfer (CT) dynamics across metal−molecule interfaces has important implications for performance and function of molecular electronic devices. CT times, on the order of femtoseconds, can be precisely measured using synchrotron-based core-hole clock (CHC) spectroscopy, but little is known about the impact on CT times of the metal work function and the bond dipole created by metals and the anchoring group. To address this, here we measure CT dynamics across self-assembled monolayers bound by thiolate anchoring groups to Ag, Au, and Pt. The molecules have a terminal ferrocene (Fc) group connected by varying numbers of methylene units to a diphenylacetylene (DPA) wire. CT times measured using CHC with resonant photoemission spectroscopy (RPES) show that conjugated DPA wires conduct electricity faster than aliphatic carbon wires of a similar length. Shorter methylene connectors exhibit increased conjugation between Fc and DPA, facilitating CT by providing greater orbital mixing. We find nearly 10-fold increase in the CT time on Pt compared to Ag due to a larger bond dipole generated by partial electron transfer from the metal−sulfur bond to the carbon− sulfur bond, which creates an electrostatic field that impedes CT from the molecules. By fitting the RPES signal, we distinguish electrons coming from the Fe center and from cyclopentadienyl (Cp) rings. The latter shows faster CT rates because of the delocalized Cp orbitals. Our study demonstrates the fine tuning of CT rates across junctions by careful engineering of several parts of the molecule and the molecule−metal interface

Real-Time Dynamics of Galvanic Replacement Reactions of Silver Nanocubes and Au Studied by Liquid-Cell Transmission Electron Microscopy

We study the galvanic replacement reaction of silver nanocubes in dilute, aqueous ethylenediaminetetraacetic acid disodium salt (EDTA)-capped gold aurate solutions using <i>in situ</i> liquid-cell electron microscopy. Au/Ag etched nanostructures with concave faces are formed <i>via</i> (1) etching that starts from the faces of the nanocubes, followed by (2) the deposition of an Au layer as a result of galvanic replacement, and (3) Au deposition <i>via</i> particle coalescence and monomer attachment where small nanoparticles are formed during the reaction as a result of radiolysis. Analysis of the Ag removal rate and Au deposition rate provides a quantitative picture of the growth process and shows that the morphology and composition of the final product are dependent on the stoichiometric ratio between Au and Ag